JPWO2010058732A1 - Magnetic resonance imaging apparatus and magnetic resonance imaging method - Google Patents

Abstract

In order to acquire an image with good imaging efficiency and less body motion artifacts, a measurement period for each cycle is detected based on body motion information of a plurality of cycles detected from a subject having periodic body motion, The number of echo signals to be measured is controlled according to the detected time width of the measurement period.

Description

  The present invention measures nuclear magnetic resonance (hereinafter referred to as `` NMR '') signals from hydrogen, phosphorus, etc. in a subject and images nuclear density distribution, relaxation time distribution, etc. , "MRI") related to respiratory synchronized measurement in an apparatus.

  MRI equipment measures NMR signals (echo signals) generated by nuclear spins that make up the body of a subject, especially the human body, and forms the shape and function of the head, abdomen, limbs, etc. two-dimensionally or three-dimensionally. It is a device that automatically images. In imaging, the echo signal is given different phase encoding depending on the gradient magnetic field and is frequency-encoded and measured as time-series data. The measured echo signal is reconstructed into an image by two-dimensional or three-dimensional Fourier transform.

  When the subject is imaged by the MRI apparatus, if the subject's body motion occurs during the imaging, artifacts due to the body motion are generated in the image, which becomes an obstacle to diagnosis. Respiratory motion is one type of body motion of the subject, and various methods are employed to eliminate artifacts based on respiratory motion. For example, there is a method of performing imaging in synchronization with respiration. This method uses a body motion detection sensor for monitoring respiration and a navigator echo to detect a respiration time phase, i.e., a displacement position of the abdominal wall based on respiration, and a predetermined respiration time phase, i.e. An image is reconstructed using an echo signal obtained at a predetermined displacement position. This respiratory synchronization imaging may be performed in combination with the synchronization with the electrocardiogram waveform.

  In respiration-synchronous imaging using a body motion detection sensor for monitoring respiration, a delay time until the respiration is stabilized from a trigger signal that is driven according to respiration is visually determined by the operator. Set the time when breathing is stable. Then, according to this setting, the apparatus executes a pulse sequence for a time that falls within the set respiratory stabilization time.

  As a measure for reducing respiratory artifacts mixed in an image, for example, there is a method described in Patent Document 1. In this method, application of phase encoding is controlled so that an echo signal measured at the timing when the acceleration of abdominal wall movement due to respiration becomes the lowest is arranged in a low-frequency region of k-space. The time efficiency of this method is the same as that of normal imaging without such phase encoding control, and artifacts are reduced compared to normal imaging.

JP 2008-148918

  In respiratory synchronization imaging using a body motion detection sensor for monitoring respiration, as described above, the pulse sequence for the time that falls within the set respiratory stabilization time is executed, so that it is inevitably within the respiratory stabilization time. The number of pulse sequences repeated and the number of echo signals measured are constant regardless of the respiratory cycle. Therefore, when the breathing interval of the subject is irregular, there is a possibility that the imaging time becomes long and becomes inefficient. This is because when trying to acquire an image with less body movement artifacts, it is necessary to match the execution time of the pulse sequence for one breath to the minimum breath stabilization time within the irregular breath stabilization time. This is because the number of echo signals that can be measured is reduced. Furthermore, no matter how much the pulse sequence execution time for one breath is shortened, if the echo signal measured at the timing when the breathing is disturbed due to irregular breathing, the artifact becomes conspicuous.

  In addition, in the method described in Patent Document 1, there is a possibility that an echo signal measured in an unstable breathing state may enter a low frequency region of k-space, and the degree of artifact reduction depends on the breathing state. It will end up. Furthermore, in the case of a subject with irregular breathing, it is difficult to detect the same breathing time phase in synchronization with the electrocardiogram waveform, and the image quality may not be stable and it may take a lot of time for imaging. There is. That is, the imaging time depends on the period of the electrocardiogram waveform and the respiratory stability.

  Accordingly, an object of the present invention is to acquire an image with good imaging efficiency and few body movement artifacts even in the case of irregular body movement in imaging using a magnetic resonance imaging apparatus.

  In order to achieve the above object, a magnetic resonance imaging apparatus and a magnetic resonance imaging method of the present invention perform measurement for each period based on body movement information of a plurality of periods detected from a subject having periodic body movements. A period is detected, and the number of echo signals to be measured is controlled according to the detected time width of the measurement period.

  Specifically, the magnetic resonance imaging apparatus of the present invention is based on a body motion detection unit that detects periodic body motion information of a subject who breathes freely, and a body motion information based on a predetermined pulse sequence. A measurement control unit that measures an echo signal of a predetermined phase encoding from the subject, an arithmetic processing unit that reconstructs the image of the subject using the echo signal, and a display unit that displays the image, The arithmetic processing unit detects a measurement period for each cycle based on the body movement information of a plurality of cycles, and the measurement control unit controls the number of echo signals to be measured according to the time width of the detected measurement period. It is characterized by.

  In addition, the magnetic resonance imaging method of the present invention detects a body motion detection step for detecting periodic body motion information of a freely breathing subject and detects a measurement period for each cycle based on body motion information of a plurality of cycles. And a measurement step of measuring an echo signal of a predetermined phase encoding from the subject according to body movement information based on a predetermined pulse sequence, and in the measurement step, The number of echo signals to be measured is controlled according to the time width.

  According to the magnetic resonance imaging apparatus and magnetic resonance imaging method of the present invention, it is possible to perform measurement in the same body motion time phase without depending on the stability of body motion. As a result, even if there is an irregular body movement, it is possible to acquire an image with good imaging efficiency and less body movement artifacts.

The block diagram which shows the whole structure of one Example of the MRI apparatus which concerns on this invention An example of a respiration waveform when respiration is regular and a diagram for explaining a first embodiment of respiration synchronization imaging according to the present invention An example of a respiratory waveform when breathing is irregular, and a diagram illustrating a first embodiment of respiratory synchronous imaging according to the present invention The flowchart showing the processing flow which concerns on the 1st Embodiment of this invention An example of a respiration waveform when respiration is irregular, and a diagram for explaining a second embodiment of respiration synchronization imaging according to the present invention The figure which shows an example of the histogram obtained by analyzing a respiration waveform The figure which shows an example of GUI which sets the threshold value for selecting a flat period on a respiration waveform The figure which shows the example of a display which displays the progress of the breathing synchronous imaging which concerns on this invention An example of a respiration waveform when respiration is irregular, and a diagram for explaining a third embodiment of respiration synchronization imaging according to the present invention

  Hereinafter, preferred embodiments of the MRI apparatus of the present invention will be described in detail with reference to the accompanying drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiments of the invention, and the repetitive description thereof is omitted.

  First, an overall outline of an example of an MRI apparatus according to the present invention will be described with reference to FIG. FIG. 1 is a block diagram showing the overall configuration of an embodiment of an MRI apparatus according to the present invention. This MRI apparatus uses a NMR phenomenon to obtain a tomographic image of a subject.As shown in FIG. 1, the MRI apparatus includes a static magnetic field generation unit 2, a gradient magnetic field generation unit 3, a transmission unit 5, The receiving unit 6, the arithmetic processing unit 7, and the measurement control unit 4 are configured.

  The static magnetic field generator 2 generates a uniform static magnetic field in the direction perpendicular to the body axis in the space around the subject 1 if the vertical magnetic field method is used, and in the direction of the body axis if the horizontal magnetic field method is used. Therefore, a permanent magnet type, normal conducting type or superconducting type static magnetic field generating source is arranged around the subject 1.

  The gradient magnetic field generator 3 includes a gradient magnetic field coil 9 wound in the three-axis directions of X, Y, and Z, which is a coordinate system (stationary coordinate system) of the MRI apparatus, and a gradient magnetic field power source 10 that drives each gradient magnetic field coil It consists of. The gradient magnetic field power supply 10 of each coil is driven in accordance with a command from the measurement control unit 4 to be described later, so that the subject 1 lies in the gradient magnetic fields Gx, Gy, and Gz in the X, Y, and Z directions. Applied to the static magnetic field space. At the time of imaging, a slice direction gradient magnetic field pulse (Gs) is applied in a direction orthogonal to the slice plane (imaging cross section) to set a slice plane for the subject 1, and the remaining planes orthogonal to the slice plane and orthogonal to each other are set. A phase encoding direction gradient magnetic field pulse (Gp) and a frequency encoding direction gradient magnetic field pulse (Gf) are applied in two directions, and position information in each direction is encoded in the echo signal.

  The transmitter 5 irradiates the subject 1 with a high-frequency magnetic field pulse (hereinafter referred to as “RF pulse”) in order to induce an NMR phenomenon in the nuclear spin of atoms constituting the living tissue of the subject 1. The high-frequency oscillator 11, the modulator 12, the high-frequency amplifier 13, and a high-frequency coil (transmission coil) 14a on the transmission side. The high-frequency pulse output from the high-frequency oscillator 11 is amplitude-modulated by the modulator 12 at a timing according to a command from the measurement control unit 4, and after the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 13, the subject 1 By being supplied to the high-frequency coil 14a arranged close to the RF pulse, the subject 1 is irradiated with the RF pulse.

    The receiving unit 6 detects an echo signal emitted by the NMR phenomenon of the nuclear spin constituting the living tissue of the subject 1, and receives a high frequency coil (receiving coil) 14b on the receiving side, a signal amplifier 15, and quadrature detection. And an A / D converter 17. The echo signal of the response of the subject 1 induced by the RF pulse irradiated from the high frequency coil 14a on the transmission side is detected by the high frequency coil 14b arranged close to the subject 1 and amplified by the signal amplifier 15. After that, it is divided into two orthogonal signals by the quadrature phase detector 16 at the timing according to the command from the measurement control unit 4, and each is converted into a digital quantity by the A / D converter 17 and calculated as echo data. It is sent to the processing unit 7.

  The measurement control unit 4 controls the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6 on the basis of a predetermined pulse sequence to apply the RF pulse and the gradient magnetic field pulse and to measure the echo signal. It is a control means to repeat. The measurement control unit 4 operates under the control of the CPU 8, and sends various commands necessary for collecting echo data necessary for reconstruction of the tomographic image of the subject 1 to the gradient magnetic field generation unit 3, the transmission unit 5, and the reception unit 6. To control them.

  The arithmetic processing unit 7 performs various data processing and display and storage of processing results, and includes a CPU 8, an external storage device such as the optical disk 19 and the magnetic disk 18, and a display 20. When echo data from the receiving unit 6 is input to the CPU 8, the echo data is stored in a memory corresponding to the K space in the CPU 8 (hereinafter described that the echo signal or echo data is arranged in the K space. Means that echo data is written and stored in this memory, and echo data arranged in K space is called K space data). The CPU 8 performs arithmetic processing such as signal processing and image reconstruction on the K space data, and displays the tomographic image of the subject 1 as a result on the display 20 and records it in the external storage device. To do.

  The operation unit 25 receives input of various control information of the MRI apparatus and control information of processing performed by the arithmetic processing unit 7 from an operator, and includes a trackball or mouse 23 and a keyboard 24. The operation unit 25 is disposed close to the display 20, and the operator controls various processes of the MRI apparatus interactively through the operation unit 25 while looking at the display 20.

  In addition, the MRI apparatus according to the present invention is mounted on or near the subject and receives a body motion sensor for detecting the body motion of the subject and a signal from the body motion sensor, A body motion detecting unit 26 for detecting the body motion information of the body. The body motion information detected by the body motion detection unit 26 is input to the CPU 8 via the measurement control unit 4.

  For example, as an example of a body motion detection sensor that detects a change in position of the abdominal wall surface based on respiration, use an air pressure sensor that attaches an air bellows that expands and contracts according to the abdominal wall surface to the abdomen and detects the air pressure inside the air bellows Can do. Since the bellows internal air pressure fluctuates in accordance with the expansion and contraction of the bellows, the fluctuating position of the abdominal wall surface can be indirectly detected by this air pressure. Alternatively, an ultrasonic sensor that irradiates the abdominal wall surface with ultrasonic waves and detects the fluctuation position of the abdominal wall surface from the time required to detect the reflected wave may be used.

  In FIG. 1, the high-frequency coil 14a and the gradient magnetic field coil 9 on the transmission side are located within the static magnetic field space of the static magnetic field generating unit 2 into which the subject 1 is inserted, and to the subject 1 if the vertical magnetic field method is used. Oppositely, if it is a horizontal magnetic field system, it is installed so as to surround the subject 1. The high-frequency coil 14b on the receiving side is installed so as to face or surround the subject 1.

  The nuclide to be imaged by the current MRI apparatus is a hydrogen nucleus (proton) which is a main constituent material of the subject as being widely used in clinical practice. By imaging the information on the spatial distribution of proton density and the spatial distribution of the relaxation time of the excited state, the form or function of the human head, abdomen, limbs, etc. is imaged in two or three dimensions.

<First embodiment>
Next, a first embodiment of the MRI apparatus and MRI method of the present invention will be described. In the present embodiment, a stable period of body movement is detected based on body movement information of a plurality of cycles, and the number of echo signals to be measured is controlled according to the time width of each stable period. Hereinafter, although this embodiment is described in detail by taking respiratory motion as an example of body motion, the present invention and this embodiment are not limited to respiratory motion. First, the outline of the present embodiment will be described, and then the processing flow of the embodiment will be described in detail.

The outline of the present embodiment is as follows.
When executing the present embodiment, the operator mounts the subject on the table in advance, attaches the body motion detection sensor to the subject, and the desired imaging region of the subject is positioned at the center of the magnetic field. So move the table. It is assumed that the subject is breathing freely without holding his / her breath during this preliminary preparation period and the next main measurement period. That is, the present invention and this embodiment do not force the subject to hold his / her breath.

  A signal from the body motion detection sensor is input to the body motion detection unit 26. When the desired imaging region of the subject is placed at the center of the magnetic field, the CPU 8 monitors the respiratory waveform of the subject input from the body motion detection unit 26 over a plurality of cycles before performing this measurement. Then, the respiratory waveform of multiple cycles is analyzed. Then, the CPU 8 determines a respiratory stable period to be detected from the respiratory waveform, that is, a flat period of the respiratory waveform (that is, an echo signal measurement period).

  2 and 3 show an example of the breathing waveform of the subject input from the body motion detection unit 26. FIG. FIG. 2 shows an example of a respiratory waveform 200 of a subject with regular breathing, and FIG. 3 shows an example of a respiratory waveform 300 of a subject with irregular breathing. Both show that the abdominal wall periodically moves up and down in the vertical direction as the subject periodically breathes. 2 and 3, the vertical axis represents the vertical displacement position of the abdominal wall, and the horizontal axis represents time. The upper side of the vertical axis corresponds to a state where the abdomen is inflated and the abdominal wall surface is raised, and the lower side of the vertical axis corresponds to a state where the abdomen is depressed and the abdominal wall surface is lowered. In addition, the flat period, which is a stable breathing period in which the position of the abdominal wall surface does not change so much, has almost the same time interval over a plurality of periods in FIG. 2 where respiration is regular, and 201a and 201b are flat periods. On the other hand, in FIG. 3 in which breathing is irregular, 301a, 301b, and 301c are flat periods, which are different time intervals within a plurality of periods. Further, the period between these flat periods is a non-flat period in which the abdominal wall surface is rapidly displaced.

  In the present embodiment, the number of echo signals to be measured is changed according to the length of the flat period, and phase encoding applied to the echo signals to be measured is also controlled. As a result, it is possible to measure echo signals necessary for image reconstruction more efficiently than in the past. That is, in the conventional respiratory synchronization imaging, the number of echo signals measured in one respiratory cycle was constant (for example, 4), so in the case of a subject with irregular breathing as shown in FIG. In the second flat period 301a, it takes too much time to measure a fixed number of echo signals, and in the second flat period 301b, there is not enough time to measure the same fixed number of echo signals. However, in the present embodiment, the number of echo signals to be measured is varied according to the length of the flat period. Preferably, the maximum possible number of echo signals is measured using the entire flat period. Therefore, it is possible to measure an echo signal necessary for image reconstruction using time efficiently.

(Determination method of flat period of respiratory waveform)
Next, a method for determining the flat period of the respiratory waveform will be described. First, as a first determination method, a method using a histogram representing a frequency distribution of respiratory waveform values will be described.

  The CPU 8 creates a histogram representing the frequency distribution of the values of the respiratory waveform, that is, the position frequency of the abdominal wall surface in the direction perpendicular to the abdominal wall surface (hereinafter z direction), using the respiration waveform as shown in FIG. An example of the histogram is shown in FIG. The horizontal axis of the histogram shown in FIG. 6 represents the displacement in the z direction of the position of the abdominal wall surface accompanying breathing, quantized between the lowest value and the highest value at a predetermined interval. That is, 0 (zero) corresponds to the lowest position of the abdominal wall position in the z direction, 144 corresponds to the highest position of the abdominal wall position in the z direction, and the values in between correspond to the intermediate positions of the abdominal wall surface. . The vertical axis of the histogram represents the frequency of occurrence of each position on the abdominal wall surface in the z direction within one cycle of respiration. In general, in the respiration cycle, the period when the abdominal wall position in the z-direction is low in the breathing cycle is longer than the period when the abdominal wall position in the z-direction is high in the breathing period. The abdominal wall position is more frequent.

After creating a histogram representing the frequency distribution of the values of the respiratory waveform as described above, the CPU 8 is in a range of the respiratory waveform corresponding to the state of exhaling, that is, a range where the value of the respiratory waveform is lower than the created histogram. The respiratory waveform value 601 having the highest frequency is detected. Then, the CPU 8 selects a respiratory waveform value range having a predetermined frequency as a respiratory stable state in both the left and right directions of the histogram centering on 601 and selects this respiratory stable state period as an echo signal measurement period. . Hereinafter, this period is referred to as a flat period. Specifically, the value of the respiratory waveform corresponding to the frequency at the right side from 601, that is, the larger one of the values of the respiratory waveform and having a predetermined frequency and located at a predetermined width β is set as the upper threshold 602. The value of the respiratory waveform corresponding to the frequency at the left, that is, the smaller one of the values of the respiratory waveform and having the predetermined frequency and located at the predetermined width α is determined as the lower threshold 603, and the respiratory waveform sandwiched between them is determined. The range of values is the flat period of the respiratory waveform. Note that α may be from 601 to the respiration waveform minimum value, β may be 0, and the respiration waveform minimum value to 601 may be the flat period of the respiration waveform.
The above is the description of the first determination method of the flat period of the respiratory waveform.

Next, a second method for determining the flat period of the respiratory waveform will be described. The second determination method is a method in which the operator selects the flat period directly on the respiratory waveform. FIG. 7 shows an example of a GUI (graphical user interface) for selection of a flat period of the respiratory waveform by the operator. On the respiration waveform 700 displayed on the display 20, a bar 701 representing an upper threshold for selecting a flat period range and a bar 702 representing a lower threshold are superimposed and displayed. The operator uses the trackball or mouse 23 to adjust and set the vertical positions of these bars 701 and 702, thereby setting an upper threshold and a lower threshold, respectively. The CPU 8 selects a period during which the value of the respiratory waveform is a value between the set upper and lower thresholds as a flat period. The lower limit threshold 302 may be the respiratory waveform minimum value, and the respiratory waveform minimum value to the upper threshold 701 may be the flat period of the respiratory waveform.
The above is the description of the second determination method of the flat period of the respiratory waveform.

  The threshold for detecting the flat period may be determined by combining the above two methods. For example, the threshold value determined by the first method may be readjusted by the second method. In that case, for example, each threshold value determined by the first method is displayed as the initial position of the upper and lower limit threshold bars 701 and 702 in the second method.

(Main measurement)
Next, respiratory synchronized measurement of a subject who breathes freely will be described. Based on the imaging conditions set by the operator, the measurement control unit 4 repeatedly executes the pulse sequence with a repetition time (TR) set as one of the imaging conditions. The repetition of this pulse sequence is preferably continued not only during the flat period but also during the non-flat period. Especially in SSFP (Steady State Free Precision) type pulse sequences that measure echo signals under steady state, it is necessary to continuously repeat the pulse sequence with a short repetition time (TR) in order to maintain the steady state. It is preferable to continue the pulse sequence with the same repetition time (TR) even during periods other than the flat period.

  In a state where the pulse sequence is being executed, the CPU 8 continuously receives the respiratory waveform from the body motion detection unit 26, and based on each threshold value set or selected as described above, the CPU 8 Detect start time and end time as needed. The start time of the flat period is the time when the respiration waveform falls outside the upper and lower threshold range, and the end time of the flat period is the time when the respiration waveform falls outside the upper and lower threshold range. is there.

  Then, when detecting the start time of the flat period, the CPU 8 instructs the measurement control unit 4 to start or restart the measurement of the echo signal. Then, the measurement control unit 4 starts or restarts the measurement of the echo signal by the pulse sequence based on the imaging conditions set in advance in accordance with the measurement start or restart instruction from the CPU 8. During the measurement of the echo signal, the CPU 8 instructs the measurement control unit 4 to stop measuring the echo signal when detecting the end point of the flat period. Then, the measurement control unit 4 interrupts the measurement of the echo signal according to the measurement interruption instruction from the CPU 8. The CPU 8 starts and restarts the measurement of the echo signal to the measurement control unit 4 and instructs the interruption until the measurement of the echo signal necessary for image reconstruction is completed. This is done for each detection. When the measurement of the echo signal necessary for image reconstruction is completed, the CPU 8 instructs the measurement control unit 4 to end the measurement, and the measurement control unit ends the execution of the pulse sequence according to the measurement end instruction from the CPU 8. .

  On the other hand, the measurement control unit 4 measures an echo signal based on the same pulse sequence in each flat period. At this time, the measurement control unit 4 controls the number of echo signals to be measured according to the length of each flat period. Specifically, the measurement control unit 4 measures many echo signals during a long flat period and measures few echo signals during a short flat period. Preferably, the measurement control unit 4 measures the maximum possible number of echo signals in each flat period using the entire period regardless of the length of the flat period. As a result, the number of echo signals to be measured varies depending on the length of each flat period.

  In addition, the measurement control unit 4 measures the echo signal by changing the phase encoding of the pulse sequence in each flat period. Specifically, in the measurement of the echo signal in each flat period, the measurement control unit 4 stores the measured phase encoding, and in the measurement of the echo signal in the next flat period, an unmeasured phase encoded echo Control application of phase encoding to measure the signal. For example, at the time of interruption of the measurement of the echo signal due to the end of the flat period, the measurement control unit 4 stores the phase encoding applied at the time of the measurement of the last echo signal, and at the time of restarting the measurement of the next flat period, The measurement control unit 4 restarts the measurement of the echo signal from the phase encoding next to the last stored phase encoding. That is, the measurement of the echo signal in the current flat period is performed by taking over the measurement of the echo signal in the previous flat period. As described above, the measurement control unit 4 does not overlap the phase encoding applied to the echo signal measured in each flat period, and measures all the phase encoded echo signals necessary for image reconstruction. Control application of phase encoding during each flat period.

  FIG. 2 shows an example of echo signal measurement during each flat period of a subject with regular breathing. The vertical line represents the application timing of the excitation pulse in the pulse sequence. The same applies to FIGS. 3, 5, and 9 to be described later. The echo signal 202a measured in the non-flat period before the flat period 201a is discarded. In the flat period 201a, the seven echo signals 203a of the phase encodings 1 to 7 are measured, and the echo signal 202b measured until the next flat period 201b is discarded. Further, since the flat period 201b has the same period width as the previous flat period 301a, the phase encoding measured in the flat period 201b is 8 to 14 and the same 7 echo signal 203b. The same applies thereafter. Thus, when breathing is regular, the number of echo signals measured in each flat period is substantially the same, and the phase encoding applied to each echo signal is continuous between the flat periods. To be controlled.

  On the other hand, FIG. 3 shows an example of echo signal measurement in each flat period of a subject with irregular breathing. The echo signal 302a measured in the non-flat period before the flat period 301a is discarded. In the flat period 301a, five echo signals 303a of phase encodes 1 to 5 are measured, and the echo signal 302b measured until the next flat period 301b is discarded. Further, since the period of the flat period 301b is shorter than that of the previous flat period 301a, the measured phase encoding is only 6 and 7 two-echo signals 303b. Similarly, since the period from the flat period 301b to the next flat period 301c is long, more echo signals 302c are measured and discarded during this period. In the next flat period 301c, the four echo signals 303c of the phase encodes 8 to 11 are measured. The echo signal 302d measured in the non-flat period next to the flat period 301c is discarded. Thus, when breathing is irregular, the number of echo signals measured in each flat period is controlled corresponding to the flat period width. That is, if the flat period is short, the number of echo signals is reduced, and if the flat period is long, the number of echo signals is increased. As in the case of regular breathing, the phase encoding applied to each echo signal is controlled to be continuous during each flat period.

  In both cases of regular breathing in FIG. 2 and irregular breathing in FIG. 3, the measurement control unit 4 ends the free breath measurement after completing the measurement of all echo signals necessary for image reconstruction.

  As in the above two examples, the measurement control unit 4 repeats the measurement of the echo signal in the flat period and the discard of the echo signal in the non-flat period regardless of whether the breathing is regular or irregular. At that time, the number of echo signals to be measured is controlled according to the length of each flat period, and the application of phase encoding is controlled so that the phase encoding applied to the echo signal to be measured is continuous in each flat period. . This makes it possible to perform time-effective imaging regardless of whether the breathing is regular or irregular. In addition, since the pulse sequence is continuously executed during the non-flat period, the echo signal intensity is stabilized over a long period of time, and the image quality is improved.

In the above explanation, an example in which the measurement of the echo signal is continued even when the respiratory waveform is non-flat has been described. However, when the respiratory waveform is non-flat, the measurement of the pulse sequence and the echo signal is interrupted without being continued. May be.
The above is the description of the outline of the present embodiment.

(Measurement progress display)
Next, display of the progress of measurement will be described. While the breathing synchronization measurement of the subject who is breathing freely is continued, the progress of the measurement may be displayed in an easy-to-understand manner to the operator, preferably also to the subject. The display content may be, for example, the ratio of measured phase encoding to the total phase encoding, the number of measured / unmeasured phase encodings, the estimated value of the remaining imaging time, and the like. By displaying at least one of these values on the display 20 or the like, it is possible to easily show the progress of the respiratory synchronization measurement to the operator.

  FIG. 8 shows an example of progress display of respiratory synchronous measurement. FIG. 8 (a) shows an example in which the measured number of phase encodings (802) is displayed on the left and right with “/” between the total number of phase encodings (801). FIG. 8B shows an example in which the actual measurement elapsed time (804) and the total measurement time (803) predicted from the total number of phase encodings are displayed on the left and right with “/” in between. Fig. 8 (c) shows an example of displaying the proportion of the measured phase encoding number corresponding to the total number of phase encodings with a progress bar, and the entire bar corresponding to the total number of phase encodings is represented as the proportion of the measured phase encoding number. An example of filling with different color bars corresponding to is shown.

(Processing flow)
Next, the processing flow of this embodiment will be described. The processing flow of this embodiment is composed of two steps, a preliminary preparation processing flow and a main measurement processing flow. FIG. 4 is a flowchart showing two processing flows of this embodiment. Hereinafter, the processing of each step will be described.

  In the pre-preparation process flow before the main measurement, first, the operator mounts the subject on the table, attaches the body motion detection sensor to the subject, moves the table, and moves the subject to the desired condition. The following steps are executed in a state where the imaging part is arranged at the center of the magnetic field.

  In step 401, the CPU 8 monitors the breathing waveform of the subject breathing freely input from the body motion detection unit 26 for a certain period of time.

  In step 402, the CPU 8 analyzes the respiration waveform and determines a threshold value for detecting a flat period of the respiration waveform. The flat period may be determined by either the above-described method using the histogram as the first method or the method directly selected by the operator as the second method. Alternatively, the threshold value determined by the first method may be readjusted by the second method.

  When the advance preparation is completed as described above, the process proceeds to the process flow 403 to 410 of the main measurement described below.

In step 403, based on the imaging conditions set by the operator, the measurement control unit 4 repeatedly executes a pulse sequence at a repetition time (TR) and starts measuring an echo signal.
In step 404, the CPU 8 obtains the breathing waveform value of the subject input from the body motion detection unit 26, that is, the displacement position of the abdominal wall surface.

  In step 405, the CPU 8 determines whether the value of the respiratory waveform belongs to the flat period or the non-flat period based on the threshold values determined in step 402. If it is determined that it belongs to the flat period, the process proceeds to step 406. If it is determined that it belongs to the non-flat period, the process proceeds to step 408.

  In step 406, since the measured echo signal is measured in a flat period, the CPU 8 sets the echo signal to a k-space position corresponding to the phase encoding applied to the echo signal in order to adopt the echo signal for image reconstruction. Place echo data. Further, as described above, the CPU 8 may update the measurement progress display according to the measured number of phase encodings.

  In step 407, the measurement control unit 4 advances the phase encoding to the next step.

  In step 408, since the measured echo signal is measured in the non-flat period, the CPU 8 discards the echo signal without using it for image reconstruction.

In step 410, the CPU 8 determines whether or not an all-phase encoding echo signal has been measured. If the all-phase encoding echo signal has been measured, the CPU 8 ends this measurement processing flow and measures the all-phase encoding echo signal. If not, return to Step 404.
The above is the description of the processing flow of the present embodiment.

  In the description of the present embodiment, the respiratory waveform has been described as an example. However, the present invention can be similarly applied to measurement using other biological information such as an electrocardiographic waveform and a pulse wave.

  As described above, according to the MRI apparatus and the MRI method of the present embodiment, image reconstruction is performed using only echo signals measured during the flat period of the respiratory waveform, regardless of whether the breathing is regular or irregular. Therefore, even if the breathing is irregular, artifacts based on the irregular breathing can be suppressed and a high-quality image can be acquired. In addition, the number of echo signals to be measured is controlled according to the length of the flat period of the respiratory waveform, and when the measurement extends over a plurality of flat periods, the phase encoding applied to the echo signal measured in each flat period The application of phase encoding is controlled so as not to overlap. As a result, imaging can be performed under free breathing without forcing the subject to hold his / her breath, and the imaging efficiency is improved regardless of whether the breathing is regular or irregular. It becomes possible to minimize.

<Second Embodiment>
Next, a second embodiment of the MRI apparatus and MRI method of the present invention will be described. In this embodiment, the respiratory synchronization measurement of the present invention is applied to multi-slice imaging. The difference from the first embodiment described above is multi-slice imaging, in which echo signal measurement in a flat period is performed with priority given to measurement of echo signals of the same phase encoding from each slice. That is, within one repetition time (TR), the phase encoding is the same and the echo signal from each slice is measured. Hereinafter, only differences from the first embodiment will be described with reference to FIG. 5, and description of the same points will be omitted. FIG. 5 shows an example in which three multi-slice imaging is performed.

Prior preparation before this measurement is the same as that in the first embodiment described above, and a description thereof will be omitted.
In this measurement, when the respiratory waveform is a flat period, the measurement control unit 4 measures the echo signal for each slice with the same phase encoding in one repetition time (TR). For example, as shown in FIG. 5, in the flat period 501a, the measurement control unit 4 repeats the multi-slice sequence three times, measures the echo signal of phase encoding 1 for each slice at the first repetition time (TR), and The echo signal of encoding 1-1 to phase encoding 3-1 is measured. Here, the first number means a slice number, and the next number means a phase encoding number. In the second repetition time (TR), the measurement control unit 4 measures the next phase encode 2 echo signal for each slice, and measures the phase encode 1-2 to phase encode 3-2 echo signals. In the third repetition time (TR), the measurement control unit 4 measures the echo signal of the next phase encode 3 for each slice, and measures the echo signal of the phase encode 1-3 to the phase encode 3-3.

  In the next flat period 501b, the measurement control unit 4 measures the echo signal of the phase encode 4, which is the next step of the phase encode last measured in the flat period 501a, for each slice, and the phase encode 1-4 to phase Measure the echo signal of encode 3-4. Note that it is not necessary to repeatedly measure echo signals having the same slice number and phase encoding during each flat period, so that the slice number and phase encoding need not be continuous.

  Hereinafter, in the same manner, the measurement control unit 4 repeats the measurement of the echo signal of the same phase encoding for each slice in each subsequent flat period with the repetition time (TR) by changing the phase encoding, and for each slice. The measurement of the echo signal in the flat period is repeated until the measurement of the phase encoded echo signal is completed.

  Note that the measurement control unit 4 repeats the multi-slice sequence even in the non-flat period, but the phase encoding at that time may be any or may not be applied. This stabilizes the signal strength of the echo signal and improves the image quality. In addition, the echo signal of each slice measured in the non-flat period, before the flat period 501a, between the flat period 501a and the next flat period 501b, and after the flat period 501b is adopted for image reconstruction. It is destroyed without

  As described above, according to the MRI apparatus and MRI method of the present embodiment, the same effects as those of the first embodiment can be obtained even in multi-slice imaging. In other words, images are reconstructed using only the echo signal measured during the flat period of the respiration waveform for each slice, regardless of whether the respiration is regular or irregular. It is possible to obtain a high-quality image for each slice efficiently.

<Third embodiment>
Next, a third embodiment of the MRI apparatus and MRI method of the present invention will be described. In the present embodiment, the respiratory synchronization measurement of the present invention is applied to three-dimensional imaging. In the three-dimensional imaging, instead of changing the slice position in the multi-slice imaging, after the volume is excited, the slice encoding is applied independently of the phase encoding in the slice direction, and the position information in the slice direction is encoded into the echo signal. The difference from each of the above-described embodiments is three-dimensional imaging, in which echo signal measurement during each flat period is performed so that slice encoding and phase encoding do not overlap, that is, at least slice encoding and phase encoding. The point is that the application of each encoding is controlled so that one is different. Hereinafter, only differences from the above-described embodiments will be described with reference to FIG. 9, and description of the same points will be omitted. FIG. 9 shows an example of performing three-dimensional imaging with a slice encoding number of 4.

Prior preparation before this measurement is the same as that in the first embodiment described above, and a description thereof will be omitted.
In this measurement, when the respiration waveform is a flat period, the measurement control unit 4 measures a predetermined slice encoding and phase encoding echo signal in one repetition time (TR), and the slice encoding and phase for each repetition. The echo signal is measured by changing at least one of the encoding. FIG. 9 shows an example in which the unit for repeating the measurement of the echo signal by changing the slice encoding with the phase encoding fixed is changed by changing the phase encoding. That is, when repeating the pulse sequence with the repetition time (TR), the measurement control unit 4 changes the slice encoding loop to the inside, and provides a loop for changing the phase encoding outside this loop to apply both encodings. To control. Conversely, the unit in which the slice encoding is fixed and the phase encoding is changed to repeat the echo signal measurement may be repeated by changing the slice encoding.

  Further, the measurement control unit 4 controls the application of each encoding so that at least one of the slice encoding and the phase encoding is different in each flat period. FIG. 9 shows an example in which the measurement control unit 4 controls each encoding so that the measurement order of slice encoding and phase encoding continues in each flat period. Since at least one of the encodings is different in each flat period, it is not necessary to be continuous.

  Specifically, in the three-dimensional imaging shown in FIG. 9, the measurement control unit 4 repeats the three-dimensional pulse sequence in the flat period 901a as a first loop to change the slice encoding by fixing the phase encoding to 1. Measure four echo signals of encoding 1-1 to 4-1. Here, the first number represents slice encoding, and the second number represents phase encoding. Next, a loop for changing the phase encoding to 2 and changing the slice encoding is repeated, and 4 echo signals of encodings 2-1 to 2-4 are measured. In the same manner, the measurement control unit 4 measures up to the echo signal of encode 3-3 according to the period width of the flat period 901a, and totals 11 echo signals of encode 1-1 to 3-3 (903a ).

  In the next flat period 901b, the measurement control unit 4 starts measuring the echo signal from the next encode 4-3 of the last encode 3-3 applied in the previous flat period 901a. Next, a loop for changing the phase encoding to 4 and changing the slice encoding is repeated, and 4 echo signals of encodes 1-4 to 4-4 are measured. Next, the loop for changing the slice encoding by changing the phase encoding to 5 is repeated, and two echo signals of encoding 1-5 to 2-5 are measured. That is, the measurement control unit 4 measures the echo signals from the encoding 4-3 to 2-5 according to the period width of the flat period 901b, and 7 echo signals of the encoding 1-1 to 3-3 in total. (903b) is measured. The same applies to the subsequent flat periods.

  Note that the measurement control unit 4 repeats the three-dimensional pulse sequence even in the non-flat period, but slice encoding and phase encoding at that time may be either, or may not be applied. This stabilizes the signal strength of the echo signal and improves the image quality. In addition, each echo signal measured in the non-flat period, before the flat period 901a, between the flat period 901a and the next flat period 901b, and after the flat period 501b is adopted for image reconstruction. It is destroyed without.

  As described above, according to the MRI apparatus and the MRI method of the present embodiment, the same effects as those of the first embodiment described above can be obtained even in three-dimensional imaging. In other words, images are reconstructed using only echo signals measured during the flat period of the respiration waveform, regardless of whether the respiration is regular or irregular. Therefore, artifacts based on respiration are suppressed and time is efficiently used. It becomes possible to acquire high-quality 3D images.

  The above is description of each embodiment of the MRI apparatus and MRI method of this invention. However, the MRI apparatus and the MRI method of the present invention are not limited to the contents disclosed in the description of the above embodiments, and can take other forms based on the gist of the present invention.

  For example, in each of the above-described embodiments, the example in which the flat period of the body motion waveform is selected as the measurement period of the echo signal has been described, but the present invention is not limited to the flat period, and a desired period of the body motion waveform is selected. You can choose. If there is another period in which the body movement is stable, such a stable period may be selected. For example, in breathing motion, a period of inhalation is possible. What is necessary is just to set a threshold value so that a desired period can be selected.

  1 subject, 2 static magnetic field generation system, 3 gradient magnetic field generation system, 4 sequencer, 5 transmission system, 6 reception system, 7 signal processing system, 8 central processing unit (CPU), 9 gradient magnetic field coil, 10 gradient magnetic field power supply , 11 High-frequency transmitter, 12 Modulator, 13 High-frequency amplifier, 14a High-frequency coil (transmitting coil), 14b High-frequency coil (receiving coil), 15 Signal amplifier, 16 Quadrature detector, 17 A / D converter, 18 Magnetic disk , 19 Optical disc, 20 Display, 21 ROM, 22 RAM, 23 Trackball or mouse, 24 Keyboard, 51 Gantry, 52 Table, 53 Housing, 54 Processing device

Claims (15)

A body motion detector for detecting periodic body motion information of a subject breathing freely;
Based on a predetermined pulse sequence, in accordance with the body movement information, a measurement control unit that measures an echo signal of a predetermined phase encoding from the subject; and
An arithmetic processing unit for reconstructing the image of the subject using the echo signal;
A display unit for displaying the image;
A magnetic resonance imaging apparatus comprising:
The arithmetic processing unit detects a measurement period for each cycle based on the body motion information of a plurality of cycles,
The magnetic resonance imaging apparatus, wherein the measurement control unit controls the number of echo signals to be measured in accordance with the detected time width of the measurement period. In the magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the measurement control unit varies the number of echo signals to be measured according to a period width of the measurement period. In the magnetic resonance imaging apparatus according to claim 1,
The measurement control unit is characterized in that the number of echo signals measured in the first measurement period is larger than the number of echo signals measured in the second measurement period having a shorter period width than the first measurement period. Magnetic resonance imaging device. In the magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the measurement control unit changes the phase encoding of the echo signal measured in the first measurement period and the phase encoding of the echo signal measured in the second measurement period. In the magnetic resonance imaging apparatus according to claim 1,
The measurement control unit controls the phase encoding so that the phase encoding of the echo signal measured in the first measurement period and the phase encoding of the echo signal measured in the second measurement period are continuous. Magnetic resonance imaging device. In the magnetic resonance imaging apparatus according to claim 1,
The measurement control unit continuously executes a predetermined pulse sequence in the measurement period and other non-measurement periods, and discards the echo signal measured in the non-measurement period without using it for the reconstruction of the image. A magnetic resonance imaging apparatus. In the magnetic resonance imaging apparatus according to claim 1,
The magnetic resonance imaging apparatus, wherein the measurement control unit measures the echo signal by executing a predetermined pulse sequence only during the measurement period. In the magnetic resonance imaging apparatus according to claim 1,
The body motion detection unit detects a waveform representing a temporal displacement of a desired position of the subject,
The arithmetic processing unit creates a histogram of waveform values based on the waveforms of the plurality of periods, and detects a measurement period for each period based on the histograms . The magnetic resonance imaging apparatus according to claim 8,
The arithmetic processing unit determines a threshold value for the waveform value for detecting the measurement period based on the histogram, and determines a measurement period for each period based on the threshold value for the waveform value. A magnetic resonance imaging apparatus characterized by detecting. The magnetic resonance imaging apparatus according to claim 9,
A magnetic resonance imaging apparatus characterized in that a period between threshold values of the waveform value is a period that is substantially flat compared to other waveform regions. In the magnetic resonance imaging apparatus according to claim 1,
The body motion detection unit detects a waveform representing temporal variation of a desired position of the subject,
The display unit displays a threshold value setting unit that receives a setting of a threshold value for the value of the waveform for detecting the measurement period, superimposed on the waveform,
An input unit that receives an operation of the threshold setting unit;
The said arithmetic processing part detects the measurement period for every said period based on the threshold value set via the said threshold value setting part, The magnetic resonance imaging apparatus characterized by the above-mentioned. In the magnetic resonance imaging apparatus according to claim 1,
The display unit displays information representing at least one of a measured number of phase encodes or a measured elapsed time. In the magnetic resonance imaging apparatus according to claim 1,
A multi-slice sequence that measures echo signals from a plurality of slices during the repetition time of the pulse sequence;
The magnetic resonance imaging apparatus, wherein the measurement control unit repeats measurement of echo signals having the same phase encoding for each slice while changing the phase encoding in each measurement period. In the magnetic resonance imaging apparatus according to claim 1,
The pulse sequence is a pulse sequence for performing three-dimensional imaging,
The magnetic resonance imaging apparatus, wherein the measurement control unit controls application of the slice encoding and the phase encoding so that at least one of the slice encoding and the phase encoding is different in each measurement period. A body motion detection step for detecting periodic body motion information of a subject breathing freely;
Detecting a measurement period for each cycle based on the body motion information of a plurality of cycles;
Based on a predetermined pulse sequence, in accordance with the body movement information, a measurement step of measuring an echo signal of a predetermined phase encoding from the subject,
With
In the measurement step, the number of echo signals to be measured is controlled in accordance with the detected time width of the measurement period.

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